Overview

abstract

Abstract The two source energy balance model (TSEB) can estimate evaporation ( E ), transpiration ( T ), and evapotranspiration ( ET ) of vegetated surfaces, which has important applications in water resources management for irrigated crops. The TSEB requires soil ( T S ) and canopy ( T C ) surface temperatures to solve the energy budgets of these layers separately. Operationally, usually only composite surface temperature ( T R ) measurements are available at a single view angle. For surfaces with nonrandom spatial distribution of vegetation such as row crops, T R often includes both soil and vegetation, which may have vastly different temperatures. Therefore, T S and T C must be derived from a single T R measurement using simple linear mixing, where an initial estimate of T C is calculated, and the temperature – resistance network is solved iteratively until energy balance closure is reached. Two versions of the TSEB were evaluated, where a single T R measurement was used (TSEB-T R ) and separate measurements of T S and T C were used (TSEB-T C - T S ). All surface temperatures ( T S , T C , and T R ) were measured by stationary infrared thermometers that viewed an irrigated cotton ( Gossypium hirsutum L.) crop. The TSEB- T R version used a Penman–Monteith approximation for T C , rather than the Priestley-Taylor-based formulation used in the original TSEB version, because this has been found to result in more accurate partitioning of E and T under conditions of strong advection. Calculations of E , T , and ET by both model versions were compared with measurements using microlysimeters, sap flow gauges, and large monolythic weighing lysimeters, respectively. The TSEB- T R version resulted in similar overall agreement with the TSEB- T C - T S version for calculated and measured E ( RMSE = 0.7 mm d −1 ) and better overall agreement for T ( RMSE = 0.9 vs. 1.9 mm d −1 ), and ET ( RMSE = 0.6 vs. 1.1 mm d −1 ). The TSEB- T C - T S version calculated daily ET up to 1.6 mm d −1 (15%) less early in the season and up to 2.0 mm d −1 (44%) greater later in the season compared with lysimeter measurements. The TSEB- T R also calculated larger ET later in the season but only up to 1.4 mm d −1 (20%). ET underestimates by the TSEB- T C - T S version may have been related to limitations in measuring T C early in the season when the canopy was sparse. ET overestimates later in the season by both versions may have been related to a greater proportion of non-transpiring canopy elements (flowers, bolls, and senesced leaves) being out of the T C and T R measurement view.